Shore-side power supply has been used for supplying ferries with electricity. Since many ferries are typically docked in the same position at the same quay it is easy to provide a suitable connection. Today, some other types of commercial ships, such as, cruise, container, and Ro/Ro vessels are connected to the electrical grid in ports around the world.
At sea, power generation onboard of a ship is provided by a main generator coupled with a propulsion engine combined with auxiliary engines for generating power needed by the ship. The main engine(s) of the ship are usually switched off once the ship berths at a quay and auxiliary generators take control of all the power generation onboard for hotelling, unloading and loading activities. The auxiliary engines today are running on cheap and low-quality fuel, which results in negative environmental impacts. Therefore, a shore-side power supply is recommended. Moreover, if the auxiliary generators are shut down, an external power supply is needed while ships are mooring at a quay.
A problem with power supplies from shore-side to ships is that different countries use AC power transmissions at different frequencies, e.g. 60 Hz in USA and 50 Hz in European countries, and ships travel between countries. Ships commonly use either 50- or 60-Hz systems for their equipment and also require different amounts of power, which may be in the range of some hundreds of kW up to 12-15 MW for large cruise ships. Depending on types of vessel, high connection voltages, in the range of 6-11 kV, may be needed for a vessel, for example a modern cruiser.
To convert AC power supply to a selected frequency and a power level, a group of equipment, for example, a rectifier, an inverter and a transformer, has to be installed at each berth of a quay, which imposes a considerable demand on a footprint/space for the conversion of power supply and also results in interference with activities at the berth. For example, when a container ship is moored at the berth, a crane often runs a full length of a berth for loading, unloading operations, which makes a great restriction of flexibility for an electrical connection to a moored ship